What is an ISO image?

In the context of files and programs, an "image", whether an ISO, floppy, or other media image, is simply a file that can be used as a virtually identical copy of the original media. This file not only contains individual data files; it also contains track and sector information and arranges all this information in a file system, just like disk media. Image files, unlike normal files, are usually not opened; rather, they are mounted.

An ISO image is simply a CD-ROM or DVD image saved in ISO-9660 format.

ISO images are mainly used as source files from which to create media. As an example, most distributions of Linux release ISO images of the installation media. These images are usually available from anonymous FTP servers (such as Indiana University's ftp.ussg.iu.edu server). After you download the image, you can use CD-burning software to recreate the physical CD-ROM/DVD install media.